Neutrino astronomy as a probe of physics beyond the Standard Model: Decay of sub-MeV B-L gauge boson dark matter

The U(1)B-L symmetry, the essential component in the seesaw mechanism and leptogenesis, is naturally equipped with a massive gauge boson. If this gauge boson is the dark matter, the scenario consistent with the seesaw mechanism predicts the gauge coupling to be of the order of O(10-19) for masses ≲1 MeV, dominantly decaying into active neutrinos. We stress and explore the important role of astrophysical neutrinos of energies from O(1) keV to ∼1 MeV in testing the well-motivated B-L symmetry extension to the Standard Model, which has been missed in the literature to date. Compared to other dark matter models, the neutrino flux in the sub-MeV energy range is a unique prediction in our setup and, once detected, would serve as a smoking gun for the existence of this B-L gauge boson and its role as the dark matter particle, opening new windows to tackle cosmological and astrophysical conundra.